Although precipitation is projected to increase in much of the United States with future climate change, in most locations that additional precipitation will merely accommodate rising evapotranspiration demand in response to temperature increases. Where the effect of rising evapotranspiration exceeds the effect of increasing precipitation, and where precipitation actually declines, as is likely in parts of the Southwest, water yields are projected to decline. For the United States as a whole, the declines are substantial, exceeding 30% of current levels by 2080 for some scenarios examined.

The study includes a number of maps showing how water might dry up under different scenarios. Here are ones showing projected changes in water yields in 2020, 2040, 2060, and 2080 under a somewhat middle-of-the-road scenario:

More dramatic scenarios see reservoirs such as Lake Mead and Lake Powell drying up completely.

“We were surprised to find that climate change is likely to have a much greater effect on future water demands than population growth,” Forest Service research economist Tom Brown told the Summit County Citizens Voice. “The combined effects of climate change on water supply and demand could lead to serious water shortages in some regions.”